They're added manually at the moment (or the list is refreshed manually, I should say.
One of the first things I'll do is automate that so it is refreshed on a schedule.
I have the Imgur upload plugin installed, and enabled, but when I try to upload an image, I get "Upload Error: Invalid image path". Did I install it wrong?
Edit: Just realized, I cloned the master branch and not the 5.x branch.